STUDY OF RECURRING SLOPE LINEAE ON THE MARTIAN SURFACE

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UNIVERSITÀ DEGLI STUDI DI PADOVA

dipartimento di fisica e astronomia “g. galilei”

Corso di laurea in Astronomia

Tesi di Laurea Magistrale

STUDY OF RECURRING SLOPE LINEAE ON THE MARTIAN SURFACE

Relatore: Prof.ssa Monica Lazzarin Co-relatori: Dr. Gabriele Cremonese

Dr. Maurizio Pajola

Laureando: Giovanni Munaretto Matricola: 1155202

Anno accademico 2017/2018

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Abstract

Recurring Slope Lineae (RSL) are a candidate source of liquid water possibly located on the shallow subsurface of Mars. Their study is important as these features, if really due to water, would have important implications for potential biologic processes, for resources on the Martian surface, for the climatic history of Mars, for its future human exploration and habitability and for the Planetary Protection issue, which currently forbids surface mission to land on RSL-bearing sites. Moreover, they are a poorly understood phenomenon over which the scientific community is currently debating and for which accurate modelling and interpretation do not exist yet.

In this thesis we will perform a statistical analysis of RSL on the Martian surface, which outlines as follows: in the first chapter we will review the relevant background and literature concerning the planet Mars and in the second chapter we will introduce Recurring Slope Lineae. In the third chapter we will perform a global survey and statistical study of all candidate and confirmed RSL sites discovered so far, which is the most updated and complete at the time of writing;

in addition, we will conduct a detailed study of RSL present in Raga crater.

In the fourth chapter we will present and discuss the results of the analysis of the third chapter. In the fifth chapter we will draw our conclusions and finally, in the sixth chapter we will discuss RSL as a science case to be studied with the Colour and Stereo Surface Imaging System (CASSIS) camera on board the ESA ExoMars Trace Gas Orbiter (TGO) mission, which will provide useful and unique information on their nature.

The aim of this thesis is to characterise all candidate and confirmed RSL through a statistical analysis of properties of the surface in which they form, thus providing a more extensive and complete global description of these features than what is present in literature. In addition we study the dependence of their length on time, slope and terrain orientation, extending and confirming studies performed by other authors.

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Sommario

Le Recurring Slope Lineae (RSL) sono candidate sorgenti di acqua liquida prob- abilmente collocate in prossimità della superficie di Marte. Lo studio delle RSL è importante poiché, se queste fossero legate ad acqua liquida, avrebbero im- portanti implicazioni per quanto concerne la presenza di potenziali processi bio- logici, l’utilizzo in-situ delle risorse sulla superficie Marziana, la storia climatica di Marte, la sua esplorazione umana e la questione della Protezione Planetaria, che attualmente impedisce l’atterraggio di missioni spaziali in prossimit `di siti contenenti RSL.

Inoltre le RSL sono un fenomeno ancora poco compreso, correntemente oggetto di studio da parte della comunità scientifica, per il quale non esiste ancora un’interpretazione soddisfacente.

In questa tesi viene effettuato uno studio statistico delle RSL sulla superficie di Marte che si dettaglia come di seguito: nel primo capitolo viene presentata una descrizione generale del pianeta Marte, mentre nel secondo capitolo vengono in- trodotte le Recurring Slope Lineae. Nel terzo capitolo viene effettuata un’analisi statistica, più completa e aggiornata di quanto presente in letteratura, di tutti i siti candidati e confermati di RSL. Inoltre, verrà effettuato uno studio dettagliato delle RSL collocate nel cratere Raga. Nel quarto capitolo verranno presentati e discussi i risultati del terzo capitolo. Nel quinto capitolo verranno presentate le conclusioni di questo studio. Infine, nel sesto capitolo verranno discusse le RSL come tema scientifico da studiare con la stereo camera CaSSIS, a bordo della missione ESA ExoMars/Trace Gas Orbiter, la quale potrà fornire informazioni fondamentali sulla natura delle RSL.

Lo scopo di questa tesi è caratterizzare tutte le RSL candidate e confermate at- traverso un’analisi statistica delle proprietà della superficie dove esse si formano, al fine di darne una descrizione più approfondita e completa di quanto presente in letteratura. Oltre a ciò, viene eseguito uno studio di alcune proprietà fondamentali delle RSL, quali la dipendenza della loro lunghezza dal tempo, dalla stagione, dalla pendenza del terreno e dalla sua orientazione che conferma ed estende quanto presente in letteratura.

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Chapter 1 Introduction

In this chapter we will give a brief overview of the most relevant aspects of the planet Mars and discuss evidences of past and present water on the Martian surface and subsurface. The aim of this chapter is to provide a background to frame the topic of this thesis, the so called Recurring Slope Lineae (RSL), which will be discussed in the next chapter.

1.1 The planet Mars

Mars is the fourth planet from the Sun in the Solar System. It is the second smallest planet, after Mercury, having a mass of ≈0.1 M⊕ (Earth Masses, 1 M⊕ = 5.97·10²⁷ g) and a radius of ≈ 0.53 R⊕ (Earth Radius, 1 R⊕ = 6.3781·10⁶ m), resulting in a gravity acceleration of 3.7 m/s², about the 38% of that of the Earth. Mars’ rotation period is slightly longer than the Earth’s one, resulting in a slightly longer solar day (or sol) of 24 hours, 39 minutes and 35.244 seconds. Its orbit has a semi-major axis of approximately 1.52 AU and an eccentricity of 0.09, resulting in a Martian sidereal year of about 686.98 Earth solar days, or 668.5991 sols.

Mars orbits approximately 1.52 times as far from the Sun as Earth, hence it receives approximately the 43% of sunlight.Since it has an axial tilt of 25.19^◦, Mars experiences seasons, which are affected by its relatively large eccentricity.Seasons are indicated with the Solar Longitude angle, Ls, which is 0^◦ at the vernal equinox, 90^◦ at summer solstice, 180^◦ at the autumn equinox, and 270^◦ at winter solstice (Clancy et al.,2000).

Mars Earth

Radius (m) 3.391 10⁶ 6.3871 10⁶ Mass (g) 6.39 10²⁶ 5.97 10²⁷

Gravity (m/s²) 3.7 9.81

Mean pressure (mbar) 7.3 1013

Mean temperature (K) 210 287

Distance (AU) 1.52 1.0

Revolution period (solar days) 686.98 365.2422 Rotation period (solar days) 1.03 0.99

Table 1.1: Summary of most relevant parameters of the planet Mars, compared to those of the Earth.

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Internal structure and topography

The internal structure of Mars is differentiated into a dense metallic core, with an estimated radius of approximately 1794 km and composed primarily of iron and nickel (Rivoldini et al.,2011), surrounded by a silicate mantle. The Martian crust has a basaltic composition of mainly silicon, oxygen and metals such as iron, magnesium, aluminium, calcium and potassium and has an average thickness of 50 km (Wieczorek and Zu- ber,2004). The crust has a relatively large variation in surface elevation of 29429 m (Carr,2007), as we can see in figure (1.1); the lowest point is located on the floor of the Hellas basin, at−8200 m and the highest point is at the summit of Olympus Mons at 21229 m (the reference for elevations is the locus of points at which atmospheric pressure is 6.1 mbar Carr, 2007).

The most important topographic feature is the so-called global dichotomy:

the northern hemisphere is dominated by low elevations terrains, while the southern hemisphere is characterized by by high elevation ones. The relative elevation between the two hemispheres ranges from 3 to 6 km.

This dichotomy is also evident when considering crater densities, with the southern highlands being heavily cratered and the northern lowlands showing a more smooth topography with few large craters and evident signs of more recent resurfacing. In addition, the global dichotomy is expressed in terms of crustal thickness, which reaches a maximum of 58 km in the southern highlands, and a minimum of 32 km in the northern lowlands (Neumann et al.,2004). Other important topographic features are the Tharsis bulge, a relatively large volcanic plateau of approximately 5000 km in diameter and 10 km in elevation, which covers up to the 25% of the planets surface. The Tharsis bulge contains the largest known volcanoes on the Solar System, namely the Asia Mons, Pavonis Mons and Ascreaeus Mons, collectively called Tharsis Montes. Instead At the west- ern edge of the plateau, it is located the tallest volcano on the planet, the Oympus Mons. A much smaller bulge is located West of the Tharsis region and it is called the Elysium volcanic complex. Important negative topographic features are the Hellas basin, a depression of approximately 9 km located at (47^◦S,67^◦E), and the Argyre basin, a 1−2 km depression located at (50^◦S,318^◦E). All these features are depicted in figure (1.1).

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1.1. THE PLANET MARS 7

Figure 1.1: The global dichotomy is evident in this colourized elevation map:

the northern hemisphere is dominated by smooth lowlands (blue) while the souther hemisphere is dominated by heavily cratered highlands (orange).

Surface features

The Martian surface is characterized by a distinctive reddish colour, due to a fine iron oxide dust cover (Christensen et al.,2003), on which dark and bright areas are present. The latter are in correspondence of polar caps, clouds and the Hellas, Tharsis and Arabia Terra regions, where mas- sive volcanoes lie. Dark areas, instead, represent regions where winds swept dust on the surface, leaving the dark, rocky bedrock exposed. The North an South poles are covered by two 1−3 km thick polar caps made of water ice, covered by a variable layer of CO₂ ice. Their estimated volume is of approximately one million of km³ each.

Atmosphere

The atmosphere of Mars is very thin, and composed primarily of CO2

and small quantities of Nitrogen. Water vapour is present, even though in traces.

During southern spring and summer, when temperatures are at their maximum values, dust storm tend to occur at low latitudes. These dust storms may be local or grow to global scales encircling the whole planet for several weeks. During dust storms the optical depth can rise from 0.5 up to 5, obscuring the surface. One of the effects of dust storms is a net transfer of dust from the southern hemisphere to the low northern latitudes. However, the amount of dust lifted by storms is small, and is equivalent to a few micrometers spread over the whole planet.

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Surface temperatures

The very thin and dry Martian atmosphere absorb little of the radiation emitted by the Sun and Mars itself. As a consequence, surface temperatures are mostly given by the balance between the solar radiation absorbed by the surface, the emitted radiation in the infrared and the net energy flux conducted by the ground. Mean diurnal temperatures range from 150 K at the poles to 240K in the warmest regions of the southern hemisphere (Kieffer et al.,1977) and peak temperatures can reach up to 300K during summer in the southern latitudes.

1.2 Water on Mars

The presence and stability of liquid water on Mars is a very important topic because its implications on potential biologic processes and human exploration. There are compelling evidences of the presence of liquid water in the past geologic history of Mars. On the contrary, even though its presence has been recently detected on the subsurface (Orosei et al.,2018), its detection on the surface is still an open question. In this section we will discuss the stability of liquid water on the Martian surface and describe all evidences and hypotheses about its presence in both past and present Martian conditions.

1.2.1 Stability of water on Mars

Liquid water is stable only for temperatures above 273K and partial pres- sures above 6.1 mbar, and would otherwise boil or directly sublimate.

These conditions are very unlikely on the surface, as the pressure ex- erted by the atmosphere is less than 6.1 mbar for most of the planet, and temperatures rise over 273K only during southern summer for a limited time of the day. In low elevation areas, where pressure is higher, and in correspondence of low permeability and ice rich soils, water could transiently form when temperatures rise over 273 K. Moreover, if the soil has enough salts, these could lower the freezing point of ice which would melt to form the so called liquid brines (i.e. salty water). However, these conditions would occur only near noon and in the uppermost centime- tres of the soil.

1.2.2 Evidences of past liquid water

Although liquid water can exist only transiently on the surface in present Martian conditions, there are evidences that the planet may have been much wetter and warmer in the ancient past (Pollack et al.,1987) and water may have flown across its surface. Evidences of past water come from observations of classical geomorphic features attributed to water, like lakes, river valleys and deltas, from the detection of its alteration on minerals composing the basaltic Martian surface and from the analysis of Martian meteorites.

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1.2. WATER ON MARS 9

Geomorphic evidences of water

Analysis of images of the Martian surface showed the extensive presence of dendritic valley networks, like the one depicted in figure (1.2B), which were formed by the eroding action of a sustained flow of liquid water.

Imaging and altimetry of these features showed that they are common on most of the oldest Martian surface and indicate that precipitation, surface runoff (i.e flooding) and seepage of groundwater occurred as well, at least episodically, in the early Martian history (Carr,2007).

Many depressions are observed to have inlet and outlet valleys. More- over, when inlet valleys enter the depression, deltas and alluvial fans are commonly observed. These geomorphic features are characteristic of ter- restrial lakes and have been interpreted as the presence of paleolakes in the Martian past history. An example is given in figure (1.2 A).

Other signs of past flowing water are outflow channels, which appear as large, long, isolated channels lacking a dendritic network of branches which characterizes the valleys discussed above. An example is depicted in figure (1.2C). Most of outflow channels are thought to be formed by the rapid release of large volumes of water.

Other features are inverted reliefs, occurring in correspondence of river beds. When sediments are deposited on the floor of a river they undergo cementation and become relatively resistant to erosion. If later the area is buried, erosion removes the less resistant covering layers until the inner, cemented layer of sediments is exposed. The latter is resistant to erosion and will show the "negative" of the river bed. An example is shown in figure (1.2 D).

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Figure 1.2: Geomorphic evidences of water; A) blue arrows indicate the inlet and outlet valleys which breach the crater rim, suggesting that water flew into the crater from Tinto Vallis in the bottom part of the figure and flew out from the breach in the crater in the top of the figure. Adapted from Carr (2007). B) A dense drainage network, adapted from Carr (2007). C) Example of an outflow channel. D) Example of a inverted relief, indicated in blue.

Evidences from alterations of surface minerals

The Martian surface is predominantly basaltic and mostly composed of igneous minerals, like olivine, pyroxene and plagioclase feldspar, which react with water to form hydrated minerals like goethite, evaporites, phyllosilicates. The detection of all these hydrated minerals is an evidence of the presence and activity of past liquid water on the Martian surface.

Another kind of alteration is due to the upward migration of subsurface hydrothermal fluids, heated by the residual heat produced by large impacts (Abramov and Kring,2005). In particular, when water migrates through the basaltic lithosphere, its interaction results in the formation of a particular rock called serpentinite, which has been observed on Mars (Ehlmann et al.,2010).

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1.2. WATER ON MARS 11

Evidence from meteorites

Evidence of the presence of water in the past Martian history comes also from the chemical analysis of Martian meteorites, which shows the alteration of olivine by water (Treiman,2005).

1.2.3 Contemporary water

As anticipated, the low average temperature and pressure of the Martian surface prevent the formation of stable liquid water, which can form only in transient anomalous conditions. On the contrary, stable liquid water has been detected below the surface using the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere sounding) instrument by Orosei et al. (2018) in a 1.5 km deep, 20 km wide subglacial lake located in the region of Planum Australe (193^◦E, 81^◦S).

Additionally, frozen water exists in the polar caps and is abundant and widespread below the uppermost meters of Martian soil, as indicated by detection of hydrogen with the Mars Odyssey Gamma Ray Spectrometer and Neutron Spectrometer (Boynton et al.,2002;Boynton et al.2007;Wilson et al.,2018). These observation revealed that buried water ice is highly abundant poleward of ±50^◦ but it also is present, though in lower con- centrations, in several low latitude locations, like the slopes of Tharsis Montes, Elysium Mons and at the Medusae Fossae formation (Feldman et al., 2004,Wilson et al.,2018). Moreover, > 100 m thick exposed water ice sheets have been imaged poleward of±50^◦ by Dundas et al. (2018).

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Chapter 2 Recurring Slope Lineae

Recurring Slope Lineae (RSL) are narrow (0.25-5 m) dark markings found on warm (T ≥ 250K) steep (≥ 28^◦) slopes with exposed bedrock outcrops. They were first discovered by McEwen et al. (2011) using the High Resolution Imaging Science Experiment (HiRISE) camera on board the Mars Reconnaissance Orbiter (MRO) mission (McEwen et al., 2007).

Since then, they have been found only in five geographic regions of the Martian surface and named accordingly: across a southern mid-latitudes band from 30^◦ S to 60^◦ S (SML RSL), in the equatorial highlands (EQ RSL), in Chryse and Acidalia Planitiae in the northern hemisphere (CAP RSL) and finally in Valles Marineris (VM RSL), where they are mostly concentrated (McEwen et al., 2011; McEwen et al., 2013b; Stillman et al., 2014; Ojha et al., 2014; Chojnacki et al., 2016; Stillman et al., 2016; Still- man et al., 2017a). Their geographical distribution is depicted in figure (2.1). The most intriguing aspect of RSL is their activity, which starts with the appearance of dark streaks departing from bedrock outcrops when surface temperature rises above 250 - 270 K, in local early spring or late summer, and continues with a phase of incremental lengthening where the lineae elongate following the topographic gradient. Once they reach the maximum extent, they remain dark for a prolonged period of time and eventually fade in cold seasons, as temperatures drop below 250 - 270 K. This sequence repeats every year in the same locations. An example of RSL is given in figure (2.2).

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Figure 2.1: Geographical distribution of confirmed RSL overlaid on the Mars Orbiter Laser Altimeter (MOLA) colorized elevation map. SML RSL are defined as those located southern than−30^◦N, CAP RSL are those inside the Chryse and Acidalia Planitiae, in the northern hemisphere. VM RSL are located in Valles Marineris and EQ RSL are those on the equatorial highlands, from≈ −30^◦N to

≈30^◦ N.

Figure 2.2: Example of RSL, indicated by white arrows, on the slopes of Juventae Chasma (−_4.691^◦_{N, 298.577} ^◦E). This temporal sequence (from A to D) of images highlights their incremental lengthening, fading and recurrence. In panel A), obtained from HiRISE image ESP_032074_1755, RSL appear as dark markings. In panel B), obtained from HiRISE image ESP_032219_1755, the same RSL appear longer except one which faded (this behavior is peculiar of VM RSL). In panel C), obtained from HiRISE image ESP_039195_1755, RSL are absent while in panel D), obtained from HiRISE image ESP_041318_1755 they re-appear on the same locations of panel A).

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2.1. OBSERVATIONS 15

2.1 Observations

RSL sites have been mainly studied with four instruments: the HiRISE camera on board the MRO mission, which provides images at a spatial scale up to 0.25 cm/px in the near infrared, red and blue part of the elec- tromagnetic spectrum; the Compact Reconnaissance Imaging Spectrome- ter (CRISM, Murchie et al., 2007) on board the MRO mission, which pro- vides spectral cubes with 544 channels from 0.4 µm to 3.92 µm at a spa- tial scale of 18 m/px; the Thermal Emission Imaging system (THEMIS, Christensen et al., 2004) on board the Mars Odyssey mission, which ob- serves in the visible wavelengths from 0.42 µm to 0.86 µm at a resolution of 18 m/px and in the infrared wavelengths from 6.8 µm to 14.9 µm with 9 spectral bands at a resolution of 100 m/px; finally the Ther- mal Emission Spectrometer (TES, Christensen et al., 2001), on board the Mars Global Surveyor mission (MGS), which observes in the infrared wavelengths from 5.5 µm to 100 µm and in the visible/near infrared wavelengths from 0.3 µm to 2.9 µm at a spatial resolution of 3 km/px.

THEMIS and TES are particularly useful to derive surface temperatures.

In the following paragraphs we will review the most important results obtained with these instruments.

HiRISE observations

Due to their small spatial scale (<5 m), the only instrument with enough spatial resolution to sample RSL at the Nyquist level is the HiRISE camera, which can resolve objects down to ≈ 80 cm in size. Repeated observation with this instrument allowed to detect 93 confirmed RSL sites and 462 candidates, for which further observations and investigation are necessary. Analyzing HiRISE images and digital terrain models (DTM), different studies suggested that most RSL are relatively more numerous on the most illuminated slopes (McEwen et al., 2013b; Stillman et al., 2014; Ojha et al., 2014; Chojnacki et al., 2016; Stillman et al., 2016; Still- man et al., 2017a; Stillman and Grimm, 2018). According to these studies, SML RSL activity starts in southern spring and occurs mainly on North, North-West and West facing slopes for SML RSL, which are the most illuminated and receive the most downward infrared flux from the afternoon atmosphere; this behavior is shared also for VM and EQ RSL, which are located near the equator, but here RSL activity occurs mainly on South facing slopes during southern spring and summer and then steps to North facing slopes during the northern summer, matching the seasonality of the most illuminated, and in turn most heated, slopes. On the contrary, CAP RSL are more problematic, as they appear preferen- tially appear on South-West and West facing slopes but they do not form on South facing slopes, which are the most illuminated. However, it must be noted that to date, only 8 confirmed CAP RSL have been identified, therefore a wide statistics is still far to be accomplished. The picture is, however, more complicate as not all RSL follows these trends, like those in Juventae Chasma (McEwen et al., 2013b; Chojnacki et al., 2016; Still- man et al., 2017a). Moreover, RSL located in different geographic regions exhibit different unique features: high cadence HiRISE imaging shows that SML RSL have two pulses of activity during their active season and

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completely fade between them, even though the insolation is at his maximum (Stillman and Grimm, 2018). VM RSL activity is peculiar since RSL both fade and lengthen at the same time in the same slope orienta- tions; moreover, RSL are longer and active for a slightly longer period of time with respect to SML RSL (Stillman et al., 2017). CAP RSL do not show any of the above mentioned peculiarities, but are active for a longer period of time (Stillman et al., 2016).

Figure 2.3: RSL activity as a function of the solar longitude from Stillman and Grimm (2018). The solar longitude L_s is the angle between Mars and the Sun, measured from the Northern hemisphere spring equinox where Ls=0.

CRISM observations

In principle, CRISM spectra can be useful to understand the composition of the Martian surface characterised by the occurrence of RSL. Both liquid water and hydrated salts have absorption bands at ≈ _{1.4 µm,}

≈ 1.9 µm and ≈ 3.0 µm, within the spectral range of CRISM. Labora- tory experiments on the dehydration of soils shows that these spectral features quickly disappear, the ≈ 1.4 µm in particular, as the soil de- hydrates, while the darker albedo is retained for longer (Massé et al., 2014) because minimal quantities of water are necessary to darken the regolith. This makes the spectral detection of liquid water very challeng- ing, because it must be observed as soon as it wets the soil, while they are more easily identified by imaging, since they are dark even when their spectral firm is not detectable. To complicate the matter, RSL have a smaller spatial scale than a CRISM pixel (0.25 cm - 5 m against 18 m) and thus they are not resolved. For best sites, where there are several lineae on the same slope, the RSL can fill up to almost one CRISM pixel (Ojha et al., 2015). The analysis of CRISM spectra conducted on 4 sites by Ojha et al. (2015) showed absorption features at ≈1.48 µm, ≈1.9 µm

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2.1. OBSERVATIONS 17

and ≈3.0 µm correlated with RSL-dense pixels, while only the ≈_{1.9 µm} and ≈3.0 µm absorptions were detected on RSL-free pixels. An example of such spectra is shown in figure (2.4). Since these detection are only from images with wide RSL, they have been attributed to their areally extensive presence. This suggested that RSL-dense pixels are more hydrated than RSL-free pixels, hence meaning that either hydrated material is transported by RSL or RSL hydrates the soil themselves. These hydration features, however, are too narrow to be attributed to surficial liquid water but they are consistent with a mixture of Martian soil and hydrated salts, namely Magnesium perchlorate, Magnesium chlorate and Sodium perchlorate. This detection is of critical importance because it suggests the contemporary presence of water and hydration of salts on the surface of Mars (Ojha et al., 2015).

Figure 2.4: Figure (3) of Ojha et al. (2015); a)RSL on the central peak of Hale crater, from HiRISE image ESP_032416_1440 (IRB channel). The white box represents the location of CRISM pixels, with errors. b) IR spectrum of the RSL inside the CRISM pixel of panel a) (blue), spectrum of a RSL-free region (green) and their ratio (red). Absorptions at 1.48 and 1.90 µm are evident.

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THEMIS and TES

The THEMIS and TES instruments are particularly useful to study the RSL because they provide the IR observations. Despite their pixel scale of 100 m/px and 3km/px prevents the study of single lineae, they have been used to assess their thermal behavior and their water content.

Thermal behavior

A thermal analysis of RSL-bearing slopes can be performed using THEMIS and TES data, but it must be noted that the contribution of all slopes inside each pixel are averaged, and thus the real surface temperature can be higher or lower than what is estimated, depending on the slope azimuth.

Another caveat is that since THEMIS observes from 2:00 to 4:00 PM, NW- facing slopes will have a near-peak temperature while other slopes al- ready reached their peak temperature earlier in the day and will thus exhibit a systematically lower temperature. Instead, TES observations are conducted from 1:00-2:00 PM and are more indicative of the peak temperatures, although averaged over 3 km. Moreover, THEMIS and TES observations are not synchronized with HiRISE ones, so the surface temperature of RSL-bearing slopes observed with HiRISE must be in- terpolated from THEMIS measurements. Considering this caveats, thermal analysis of RSL-bearing slopes showed that RSL lengthen only when mid-afternoon surface temperatures rises over 250-270 K, and they stop and fade when surface temperatures drop below these limits. (McEwen et al.,2013b; Stillman et al.,2014; Ojha et al.,2014). The temperature dependence for SML RSL is depicted from figure (2.5).

Figure 2.5: Temperature dependence for SML RSL, obtained from Figure (3) of Stillman et al. (2014), as a function of season (L_s) and Local True Solar Time (LTST). The blue horizontal line represents the melting point of pure water at T = 273 K. All RSL start lengthening above 273K and stop lengthening and eventually fade below this value.

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2.2. FORMATION MECHANISMS 19

Water content

THEMIS data have been used by Edwards and Piqueux (2016), in con- junction with a thermal model and a wet regolith model, to assess the water content of RSL. Water, filling the space between grains in the regolith, creates a continuous medium in which the heat can flow, increasing the regolith’s thermal conductivity and inertia and, ultimately, affecting the temperature. With a thermal model it is possible to link the observed surface temperature to the regolith thermal inertia and then, with a wet regolith model, the associated water content can be inferred. Estimates conducted at Garni crater revealed that RSL are probably dry, having a water content consistent with that of slopes without RSL. However, it must be noted that this method is insensitive to water contained in hydrated salts, as that detected by Ojha et al. (2015). However, other authors showed how these results were obtained using THEMIS pixels without a significant coverage of RSL (Stillman et al.,2017a), thus criticiz- ing the conclusions that RSL-bearing slopes have the same water content of RSL-free slopes.

2.2 Formation mechanisms

The observational facts outlined in the previous sections allow us to de- fine RSL as features which exhibit the following traits:

• Darker albedo with respect to their surroundings;

• Annual recurrence in the same places;

• Incremental lengthening followed by a dark, static, phase and sub- sequent fading;

• Occur when mid-afternoon temperature are greater than 250−_270K.

According to this definition, RSL are distinct from other processes that appear as dark features, which can misinterpreted as RSL:

• Rock falls, which appear as dark lineae like RSL but occur on a much shorter timescale, without showing incremental lengthening and annual recurrence (McEwen et al.,2011);

• Dark slope streaks, a phenomenon associated with the removal of dust which exposes the underlying darker regolith. This results in dark streaks, very similar to RSL dark markings. The removal of dust associated with dark slope streaks can be produced by winds or by the downfall of dust itself on over-steepened slopes. (Hansen et al.,2011) Although they resemble RSL, they are a distinct phenomenon as they are found in places with a high dust content where no RSL are observed. They are much bigger than RSL and do not show annual recurrence: an example is depicted in figure (2.6A);

• Gullies, which are a widespread phenomenon on the Martian surface, and are generally due to the downslope transport of sediments

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triggered by the defrost of CO2ice (Malin and Edgett,2000; Hansen et al.,2011). They can have different forms, as depicted in figures (2.6B, C) and are traditionally divided in classic Gullies, which have an alcove-channel-apron shape and linear Gullies, which resemble RSL as they appear as dark markings. Together with RSL, Gul- lies are the main active process occurring on the present Martian surface. What distinguishes linear gullies from RSL is their fading timescale, which is greater than 1 year, at least.

Figure 2.6: White arrows points to: A) Dark slope streaks in Arabia Terra (3.680^◦N, 26.069E) from HiRISE image ESP_012410_1835. B) Classic gullies on the walls of a southern crater located in (−40.304^◦N, 196.831^◦E) from HiRISE image PSP_005930_1395. C) Linear gullies in Hellas Planitia (−_48.329^◦_{N, 73.703}^◦_E) from HiRISE image ESP_037014_1315. D) Recurring Slope Lineae in Juventae Chasma (−4.691^◦N, 298.577^◦E). For A), B) and C) North is up; for D) North is down. All images are publicly available at https://www.uahirise.org/.

Several hypotheses have been put forward to explain the nature of RSL.

We will here divide them in two classes: ”wet models”, which invoke liquid water, and ”dry models” which are essentially dry flows of debris or granular material that envision a very limited role for any volatile.

Wet models

According to ”wet models” RSL are due to liquid water, with eventually a salt component (in this case, we speak of "brines"), which has a different origin depending on the model. Activity starts in hot seasons, as soon as temperatures rise over the freezing point of water or brines. This water would darken the regolith and eventually evaporate, resulting in the fading of RSL in cold seasons, as soon as temperatures drop. Activity

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starts with a incremental lengthening phase, as observations shows, in which the water input from the source is greater than the evaporative losses. However, as temperature rises and RSL area increases, the latter becomes greater and greater until it balances the input from the water source, resulting in an "equilibrium phase"; in this phase RSL no longer lengthen but remain dark. Eventually evaporation dominates when temperatures drop, freezing all the water and quenching the input; at this stage the dark lineae fade away quickly. Wet models differ in the role of water and its origin; in the following paragraphs we will describe all the relevant hypotheses which have been put forward up to now.

Deliquescence of water vapour by hygroscopic salts

Deliquescence is the absorption of atmospheric water vapour is by hygroscopic salts. According to the scenario, this absorbed water vapour is released to form RSL (McEwen et al.,2011; McEwen et al.,2013a; McEwen et al.,2015). Deliquescence would work very well in polar and high latitude regions, where the relative humidity is high, but has been proven to unlikely explain RSL because their activity do not correlate with peak water vapour concentration and the quantity of trapped water would be very low in the present Martian atmosphere.

Wet debris flows

In this case RSL would be originated from a surface flow or seep of wet debris triggered by the melting of a frozen aquifer during hot seasons (McEwen et al., 2011). Darkening here is not due to wetting of the surface, because water would rapidly evaporate, but to the change in grain size and roughness operated by the debris flow. Despite seasonality, recurrence and darkening of RSL would be explained, the fading would be problematic as it would require an opposite change in grain size, for which a viable mechanism has not been identified yet.

Briny surface flows

In this case RSL would be originated from surface flows or seeps of liquid briny water, resulting from the seasonal melting of a frozen aquifer (McEwen et al., 2011). Briny water is more stable on the Martian surface since salts can lower significantly (up to ten times) the evaporation of brines (Altheide et al.,2009), hence darkening here can be explained by both grain size sorting and wetting; fading would then be explained via evaporation and consequent dehydration of the regolith. However, even tough brines are more stable than water on the Martian surface, they can last only some hours.

Briny flows in the shallow subsurface

Unlike previous cases the flow occurs in a thin layer in the shallow (<50 cm) subsurface; the flow is powered by the melting of a frozen ice source, and it is subject to evaporative losses. Darkening is explained by mois-

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turizing of the regolith by water, and fading is due to evaporation, which dominates as soon as the water input is quenched by the freezing of the ice source. (Grimm et al.,2014). This conceptual model is shown in figure (2.7).

Figure 2.7: Conceptual model for wet-dominated RSL, from figure (2) of Grimm et al. (2014). RSL flows in a thin layer and are subject to a water input and evaporation. The flow causes the soil to saturate, which results in its darkening.

Among these "wet models", the briny subsurface flow is the most likely as it better explains observations: in particular, it gives the best expla- nation for fading. Recurrence, instead, is accomplished by assuming the presence of a briny ice source. Numerical modeling by Chevrier and Rivera-Valentin (2012) showed that brines can melt in the top cen- timeters of the subsurface at time and places when RSL are active, and remain stable against evaporation during all the the duration of RSL.

Brines are thought to have formed by the interaction between the Mar- tian hydrosphere with its reactive basaltic lithosphere, froze during the end of the warm period at the late Noachian, and are expected to be pre- served in the subsurface (Knauth and Burt, 2002). Droplets of brines have been detected on Mars on the struts of the Phoenix Mars Lander (Renno et al.,2009) and spectral analysis performed with CRISM, as detailed in section (2.1), detected the presence of hydrated salts in correspondence of RSL activity, which strongly support the presence brines at RSL sites.

Despite these facts, "wet models" share the common problem of lacking a plausible water source, which is currently investigated: Grimm et al.

(2014) estimates a water budget of 2−10 m³ per m of RSL, resulting in a water volume of 10⁴ to 10⁶ m³ of fluid for RSL in Coprates and Melas Chasma, two particularly RSL-rich regions in Valles Marineris ( Choj- nacki et al., 2016). However, these estimates assume a thickness of the wet layer which is not proven. Such water budgets are at least one order of magnitude greater than those provided by ice in the shallow subsurface (< 400 m), and this ice would not survive in equilibrium with the atmosphere since temperatures are too high; out-of-equilibrium ice, instead, would be rapidly depleted near the surface (Mellon et al.,1997).

Another option is the presence of regional groundwater basins (Stillman et al., 2014; Grimm et al., 2014); however, as some VM RSL occurs on local 100 m high topographic highs where the presence of an underlying aquifer capable of sourcing from 10⁴ to 10⁶ m³ of water is problematic, this option was deemed unlikely (Chojnacki et al., 2016). Stillman et al.

(2016) instead suggested that CAP RSL could be powered by a either a shallow or deep liquid briny aquifer which is connected to the surface via fractures or faults. During cold seasons these fractures freeze cre-

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ating and ice dam. As temperatures rise this ice dam is breached and the aquifer discharges to the surface, forming RSL. At this stage they incrementally lengthen since the discharge rate is greater than evaporation. As temperatures rises, evaporation matches the discharge rate and the lengthening slows and eventually stops. Finally, when temperatures decrease, the ice dam forms again, quenching the water input, and RSL rapidly fade. According to Chojnacki et al. (2016) this mechanism could work also in Valles Marineris, provided that viable pathways to the subsurface exists. Fractures and faults may be plausible pathways, since many bedrock outcrops from which numerous RSL are sourced are highly fractured (Watkins et al., 2014).

Dry flow models

These models envision a limited role for volatiles, like water or brines, and explain RSL as essentially water-free process. The main hypothesis, advanced by Dundas et al. (2017), is that RSL are caused by the downslope movement of fine granular material which would appear dark because of particle size and roughness effects. This hypothesis is supported by the fact that RSL have been observed to propagate only until the slope is near 30^◦ (Dundas et al., 2017) which is the angle of repose of granular flows. These idea is intriguing, because it implies a limited role for water and does not pose the problem of finding a plausible water source. How- ever, it does not explain seasonality, recurrence, fading and the detection of hydrated salts at RSL sites (Ojha et al.,2015). The fading is still un- explained, while to address seasonality and recurrence, two mechanism have been proposed: the first involves the displacement of grains caused by boiling water (Massé et al.,2016) and the second involves the displacement of grains triggered by intra-grain pressure gradients (Schmidt et al., 2017).

Wet-triggered debris flow

Laboratory experiments by Massé et al. (2016) showed that freshwater at 293 K at the top of 1 mm of regolith boils, displacing grains to form a mm-high ridge. This ridge increases the local slope angle above the angle of repose of granular material, which flows downslope. Water then flows downhill, repeating this process and producing the observed incremental lengthening. The presence of a thin film of freshwater would explain the decrease in albedo (Massé et al., 2014) and the lack of water spectral features, as they would be under the threshold of current orbital instruments (Massé et al., 2016). These model is intriguing, as it requires a very limited quantity of water, but it requires both freshwater and high temperatures for boiling. The last condition is not met for SML RSL, as their "first pulse" is observed to be active during early spring, when temperatures are colder than 273 K.

Rarefied gas-triggered debris flow

Schmidt et al. (2017) introduced a granular flow to explain RSL in Garni crater, a well-documented confirmed site in Valles Marineris (11.5^◦S,

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290.3^◦E). They invoke a Knudsen pump (Knudsen, 1910) to seasonally reduce the angle of repose of grains and trigger a dry granular flow. A Knudsen pump is a process in which, under low pressure conditions, temperature gradient are able to move particles from the hot end to the cold end. This process occurs in the top 100−200 µm of the Martian soil and despite having been advocated to explain dust-lifting processes (Wurm et al., 2008), is generally too weak to uplift grains. Instead, this process is enhanced by shadows casted by boulders, which generate strong temperature gradients close to the surface. In this case the pump is said to be "enhanced", as it exerts a greater uplifting force which opposes to gravity. This is equivalent to a situation with reduced gravity, that for granular flow translates in a reduced angle of repose (in presence of reduced gravity, the same flow can be obtained with lower slopes).

According to Schmidt et al. (2017), shadows casted by boulders at Garni crater create enhanced temperature gradients which results in enhanced pumps, that lower the angle of repose below the observed slope angle and triggers the flow. This phenomenon is also seasonal, as temperature gradients will be greater in hot seasons and lower in cold seasons. In this scenario, the relative darker albedo would be due to the sorting of the grains during the flows, while fading would be due to the aeolian deposition of the finer grains from the atmosphere. Despite the predicted decrease of the angle of repose by the Knudsen pump matches RSL activity on Garni crater (Schmidt et al.,2017), it predicts the presence of RSL also on slopes where they are not observed. Moreover, since this mechanism occurs only in correspondence of shadows, it is not clear if it can displace material incrementally over all the length of RSL, hun- dreds of meters downslope. Another flaw is that it does not explain the

"first pulse" of SML RSL, which occurs at lower temperatures. Finally, the proposed mechanism to explain the fading by grains deposition would not work for most RSL as no visual evidence of aeolian features depos- ing material has been detected at the HiRISE resolution (Stillman and Grimm, 2018).

In summary, dry granular flows are interesting because they involve zero or very limited quantities of water, which can be provided by ice in the shallow subsurface. Moreover, they can explain why RSL termi- nate on slopes≈30^◦, i.e. near the angle of repose of granular flow, while water-based flow are expected to propagate further (Dundas et al.,2017).

Stillman and Grimm (2018) argued that after the MY 28 global encircling dust storm RSL lengthened more, as a consequence of the heating induced by the storm. According to these authors, this behavior means that RSL activity, and in turn their length, is limited more by temperature than by the slope angle, even though temperature is not the only variable affecting RSL activity. This aspect is more indicative of a wet-dominated process originating RSL. Moreover, dry granular flow are criticized because they do not explain some other behaviors of RSL such as their ability to propagate over bedrock and to disappear and reappear some meters downhill, which would instead be explained by differences in capillarity by wet dominated models (2018,Stillman and Grimm).

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To conclude, the scientific community is still debating on which phenomenon might drive Recurring Slope Lineae. Two leading theories are being evaluated, one concerning dry flows of granular material and the other wet-dominated flows of briny water. To date, each model can explain some observations, but has unresolved issues. A precise evaluation of both theories on a common experimental dataset is still missing and would prove useful in establishing which model is more likely

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Chapter 3 Data Analysis

The study of Recurring Slope Lineae that we will perform in this thesis is organized in two parts: the first is detailed in section (3.1) and it is a global survey of all candidate and confirmed sites where RSL have been identified. The main aim of this part is to create the most complete catalogue of RSL to date. For all sites in the catalogue we study the global properties of the Martian surface, i.e. their elevation, slope, thermal inertia, rock abundance, dust cover index, albedo and water equivalent hydrogen. Such catalogue enables us to perform a statistical characteri- zation of the global properties of RSL sites, which is the most extensive and update one at the time of writing and it will be used to find new potential RSL sites.

The second part is detailed in section (3.2) and presents a specific case study of RSL in Raga crater, a well observed confirmed site, where we measure key properties of individual RSL such as their length as a function of time, slope and terrain orientation.

3.1 Survey of Recurring Slope Lineae and their global properties

To date RSL have been mainly identified with the HiRISE camera, i.e.

the only instrument capable of resolving such small features. RSL are defined as confirmed if they meet the definition given in (2.2) i.e. if they appear a) darker than the surroundings, b) they incrementally lengthen and c) fade and re-occur in the same places. In some cases, only a sub- set of these conditions is met; for example, the lack of repeated images can prevent the observation of the lengthening and fading phases and the annual re-occurrence; in such cases RSL are defined as candidates, as more observations are needed to discern between RSL or other features, like rockfalls or dark slope streaks.

To create a catalogue of RSL, we identified and merged all sites studied in literature (McEwen et al.,2011; Stillman et al.,Stillman et al.; McEwen et al.,2013b; Stillman et al.,2016; Chojnacki et al.,2016; Stillman et al.,2017;

2018,Stillman and Grimm). This allowed us to catalog a total of 555 sites, out of which 462 are candidates and 93 are confirmed RSL. The majority of them lies within Valles Marineris (203 candidate and 40 confirmed), followed by those in the southern mid latitudes (167 candidate

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and 40 confirmed); 84 sites are in the equatorial highlands, where 79 are candidate and 5 confirmed; finally 24 sites are in Chryse and Acidalia Planitiae, of which 16 are candidate and 8 confirmed. The geographical distribution of confirmed sites is shown in figure (2.1).

In order to have a general picture of RSL properties, we compute elevation, slope angle, thermal intertia, rock abundance, global albedo, dust cover index and the water equivalent hydrogen of RSL sites. All these quantities are here described.

Elevation and Slope angle

Elevation has been computed at RSL sites from the Mars Orbiter Laser Altimeter (MOLA,Smith et al., 2001) digital elevation model (DEM), which samples the elevation of all the Martian surface at spatial resolution of 463 m and an uncertainty of 3 m. From this DEM we computed the slope angle, at the same spatial scale, using the implementation of the Burrough and McDonnell (1999) algorithm provided by the ArcMap^® software. The elevation map is shown in figure (1.1).

Thermal Inertia

Thermal inertia represents the ability of a material in storing and conduct its internal energy. It is defined as:

I =pkρc (3.1)

where k is the thermal conductivity, with units (Wm⁻¹K⁻¹), ρ is the density (with units kg m⁻³), c is the specific heat capacity, with units ( J Kg⁻¹K⁻¹). Thermal inertia is measured in (J m⁻²K⁻¹s⁻¹²) which are often referred as ”TIU” (Thermal Inertia Units, 1 TIU = _{1 J m}⁻²_K⁻¹_s⁻¹² Putzig and Mellon, 2007).

In the context of planetary surfaces, thermal inertia measures the ability of the surface to store heat during the day and release it during the night. It depends on the properties of the surface, such as particle size, rock abundance and exposure, and it affects the surface temperature. As an example, sands have low thermal inertia, and thus can store and release heat quickly; on the contrary rocks have a high thermal inertia, so they store and release thermal energy on a longer timescale.

Thermal inertia can be derived from infrared observations; in such a case we distinguish between daytime thermal inertia, computed from observations performed during the local day, and night-time thermal inertia, computed from observations performed during the local night. The latter are usually preferred as they are less affected by temperature variations induced by the topography, which instead affect daytime observations as the most illuminated slopes are systematically hotter (Edwards and Piqueux, 2016). For each RSL site, we computed both its daytime and night-time thermal inertia from the global maps of Putzig and Mellon (2007), which have a spatial resolution of 3 km, derived from MGS-TES

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3.1. SURVEY OF RECURRING SLOPE LINEAE AND THEIR

GLOBAL PROPERTIES 29

brightness temperatures. Such maps are depicted in figures (3.1) and (3.2).

Figure 3.1: Daytime thermal inertia map from Putzig and Mellon, 2007)

Figure 3.2: Night-time thermal inertia map from Putzig and Mellon, 2007)

Rock abundance

The rock abundance (K) is the areal density of rocks. Wwe compute K from the map of Christensen (1986), shown in figure (3.3). Such map has a resolution of 60 km and it is derived from Viking Infrared Thermal Mapper (IRTM, Kieffer et al.,1972) observations. A value of K = 40%

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means that the corresponding 60 km by 60 km pixel is covered for the 40% by rocks.

Figure 3.3: Viking IRTM rock abundance map from Christensen (1986)

Global albedo

The Lambert albedo of RSL sites, which is the ratio between the emitted and incident energy fluxes, was derived from the TES albedo maps (Christensen et al., 2001), where it is computed as:

AL = ^RD

2

S cos(i) ^(3.2)

where R is the calibrated radiance, representing the energy emitted by the surface, D is the Sun-Mars distance, S is the Solar radiance integrated over the TES bolometer spectral response, representing the incident energy, and i the incidence angle (Christensen et al., 2001). Such map has a resolution of 7.5 km and is depicted in figure (3.4).

Figure 3.4: TES albedo map from Christensen et al. (2001).

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Dust Cover Index

The content of dust above the Martian surface is measured by the Dust Cover Index (DCI), developed by Ruff and Christensen (2002) from TES observations. Typical ”dusty” areas have a DCI≈0.931 while ”dust free”

areas have a DCI ≈0.97. We extracted the DCI of all RSL sites from the map of Ruff and Christensen (2002), at a resolution of 3.5 km.

Figure 3.5: Dust Cover Index map from figure (14) of Ruff and Christensen (2002). Orange, red, and white colours indicate dust covered areas, while blue and magenta areas are dust-free. Spatial resolution is 3.5 km.

Water equivalent Hydrogen

Water equivalent hydrogen is the mass fraction of H2O corresponding to a certain Hydrogen mass fraction (Feldman et al., 2004). Neutrons and gamma rays are emitted as a result of collisions between cosmic rays and atoms on the surface. When cosmic rays interact with atoms they generate neutrons at energies lower than 0.4 eV (thermal neutrons), between 0.4 eV and 0.7 MeV (epithermal neutrons) and from 0.7 to 1.6 MeV (fast neutrons) (Feldman et al., 2004). These neutrons may scatter with surrounding nuclei, exciting them and producing gamma rays, or es- cape, reaching the surface and being detected by orbital detectors. Some elements in the soil can instead capture neutrons, resulting in a decrease of the number of neutron reaching the surface. Hydrogen is particularly effective in this process, and its presence produces both a decrease of epithermal and thermal neutrons and a spectral line at 2223 KeV. Both these effect were measured by the Mars Odyssey Gamma Rays Spectrometer and Neutron Spectrometer, and related to the water content of the Mar- tian surface in terms of its water equivalent hydrogen (Boynton et al.,

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2002; Feldman et al., 2004; Wilson et al., 2018). In particular, we use the values computed by J. Wilson (Wilson et al., 2018, private communica- tion) for all RSL sites in our catalogue. These values are indicative of the water content in the uppermost meter of the surface, at resolution of approximately 300 km.

3.1.1 RSL catalogue and global properties

We here summarize our catalogue, while all rows are included in appendix A. Every row refers to a single RSL site and columns represents:

• LAT: North-positive latitude of the site, in decimal degrees;

• LON: East-positive longitude of the site, in decimal degrees

• STATUS: either "1" for confirmed sites or "0" for candidate sites;

• TYPE: either "SML" for southern mid latitudes RSL, "EQ" for Equa- torial RSL, "CAP" for RSL located in Chryse and Acidalia Planitia and "VM" for RSL located in Valles Marineris;

• H: MOLA elevation in metres. All values have an uncertainty of 3 m. (Smith et al., 2001).

• S: MOLA slope, in decimal degrees;

• TId: daytime thermal inertia, in TIU. Values have a relative uncertainty of 0.06 (Putzig and Mellon, 2007).

• TIn: night-time thermal inertia in TIU. Values have a relative uncertainty of 0.06 (Putzig and Mellon, 2007).

• K: rock abundance. Values have a relative uncertainty of 0.2 (Chris- tensen, 1986).

• A: global albedo;

• DCI: dust cover index;

• WEH: water equivalent hydrogen in units of H2O mass fraction (% wt);

Region Status Lat Lon H S TId TIn K DCI A WEH

VM 1 -15.49 308.53 -3327 22 268±16 472±28 11±2 0.141 0.9704 5 VM 1 -15.40 309.54 -1333 23 192±11 402±24 19±3 0.123 0.9702 5 VM 1 -14.70 304.30 -3622 20 52±3 345±20 6±1 0.115 0.9607 5 VM 1 -14.60 302.40 -1924 25 325±19 427±25 1±1 0.121 0.9704 5 VM 1 -14.40 304.60 -1487 28 469±28 365±21 6±1 0.112 0.9704 5 Table 3.1: First rows of the catalogue, which is reported in Appendix A.

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To summarize our catalogue, we plot the distributions of themost important variables, which will be discussed in the next section. Since distributions are far from gaussian, we compute the median to express their central tendencies. Instead their variability is expressed through the width of the central 50% of the distribution, defined by values of the 25-th and 75-th percentiles. All these values are reported in table (4.1).

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Figure 3.6: Distributions of daytime thermal inertia for (A) all candidate and confirmed RSL sites, (B) VM RSL, (C) EQ RSL, (D) SML RSL, (E) CAP RSL.

Black lines represent median values for confirmed RSL.

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Figure 3.7:Distributions of night-time thermal inertia for (A) all candidate and confirmed RSL sites, (B) VM RSL, (C) EQ RSL, (D) SML RSL, (E) CAP RSL.

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Figure 3.8:Distributions of rock abundance for (A) all candidate and confirmed RSL sites, (B) VM RSL, (C) EQ RSL, (D) SML RSL, (E) CAP RSL. Black lines represent median values for confirmed RSL.

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Figure 3.9:Distributions of dust cover index for (A) all candidate and confirmed RSL sites, (B) VM RSL, (C) EQ RSL, (D) SML RSL, (E) CAP RSL. Black lines represent median values for confirmed RSL.

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Figure 3.10: Distributions of albedo for (A) all candidate and confirmed RSL sites, (B) VM RSL, (C) EQ RSL, (D) SML RSL, (E) CAP RSL. Black lines represent median values for confirmed RSL.

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Figure 3.11: Distributions of water equivalent hydrogen for (A) all candidate and confirmed RSL sites, (B) VM RSL, (C) EQ RSL, (D) SML RSL, (E) CAP RSL.

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3.1.2 Potential RSL sites

The catalogue of RSL enabled us to identify all regions of the Martian surface having the same surface properties of our sample, which can thus be considered as potential targets to be searched for RSL. To do so we computed the ranges of all variables defined in section (3.1) for all confirmed RSL and queried all the relative maps within these ranges.

Such regions are depicted in figure (3.12).

Figure 3.12: In green, regions of the surface having surface properties within the ranges of those of all confirmed RSL sites. In red, regions with surface properties outside the ranges of those of all confirmed RSL sites. In the background, a greyscale MOLA elevation map. Regions that are not either red or green are not mapped in one or more global maps. Black triangles represent the locations of all confirmed RSL sites.

3.2 Study of RSL in Raga crater

In this section we describe the analysis of single RSL in Raga crater, a well observed confirmed site located in (−48.1N, −117.56E). Its geographical position and a HiRISE image are shown in figures (3.13A) and (3.13D).

This site was selected because it is a good compromise between extensive imagery, a large number of well visible RSL, the presence of a digital terrain model (DTM) and several orthorectified images. A DTM of a region is a dataset of elevation measurements of the region. A HiRSE DTM is derived from two images of the same region, called stereo pairs, and samples its elevation with a spatial resolution of 1 m and an uncertainty of 30 cm (Kirk et al.,2008). Orthorectified images are derived from original images, when a DTM is used to convert these in nadir pointing images.

Their usefullness is that orthorectified images have the same viewing geometry, and thus they are not affected by parallattic distortions. All these aspects, described and motivated in section (3.2.1), are essential to measure RSL key properties such as their length as function of terrain

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3.2. STUDY OF RSL IN RAGA CRATER 41

elevation, slope and aspect (i.e. the angle between the normal of the slope and the direction of the North, measured Eastwards).

Figure 3.13: A) Colorized MOLA elevation map showing the location of Raga crater (rectangle). B) Slope map. C) Aspect map. D) HiRISE image ESP_023215_1315 of Raga crater overlaid on colorized elevation map from HiRISE DTM DTEEC_014011_1315_014288_1315_A01. E) Zoomed view of dashed rectangle of panel D) with RSL represented by orange lines.

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3.2.1 Dataset

Due to the small scale of RSL (0.25-5 m wide and ≈ 100 m long) the only useful dataset available to date is the HiRISE catalog (https://

www.uahirise.org/), in which all HiRISE images and data products are stored. For our investigation, we use repeated images of Raga crater acquired between Mars Year ¹ 29 and 31 in the visible wavelengths. Al- though colour images in the infrared, red and blue part of the electro- magnetic spectrum are available, they are not useful for our study since they are restricted only to the central part of the image and do not con- tain all RSL which we want to analyse. However, different images are acquired with different observational parameters, namely incidence, il- lumination and phase; as a consequence, the same region is viewed from a different perspective for each image. This problem must be addressed when performing topographic measurements and slope analyses, as in our case, using orthorectified images. In such a case, the only factor affecting distance measurements is the cosine of the slope angle. Being this angle invariant between images, measurements made on different orthorectified images are consistent between themselves. In addition to orthorectified imagery, a digital terrain model (DTM) is necessary to measure true distances. A DTM is a dataset which provides the elevation for every point of an image. With HiRISE DTMs it is possible to sample elevation with a spatial scale of 1 m and an uncertainty of approximately 30 cm (Kirk et al., 2008). Moreover, from a DTM it is possible to compute both slope angle and aspect and convert apparent distances in true distances. In our case, all these operations are performed through the the ArcMap^®software. In our analysis, we used the following data products:

Product ID Pixel size (m) Ls (^◦) MY ESP_014011_1315_RED_A01_ORTHO.JP2 0.25 308.3 29 ESP_014288_1315_RED_A01_ORTHO.JP2 0.25 320.8 29 ESP_020947_1315_RED_A01_ORTHO.JP2 0.25 217.3 30 ESP_021514_1315_RED_A01_ORTHO.JP2 0.25 245.0 30 ESP_021870_1315_RED_A01_ORTHO.JP2 0.25 262.6 30 ESP_022226_1315_RED_A01_ORTHO.JP2 0.25 280.0 30 ESP_022437_1315_RED_A01_ORTHO.JP2 0.25 290.1 30 ESP_023004_1315_RED_A01_ORTHO.JP2 0.25 316.3 30 ESP_023215_1315_RED_A01_ORTHO.JP2 0.25 325.6 30 ESP_023782_1315_RED_A01_ORTHO.JP2 0.25 349.3 30 ESP_023993_1315_RED_A01_ORTHO.JP2 0.25 357.7 30 ESP_024204_1315_RED_A01_ORTHO.JP2 0.25 5.9 31 ESP_029149_1315_RED_A01_ORTHO.JP2 0.25 188.6 31 ESP_030428_1315_RED_A01_ORTHO.JP2 0.25 249.8 31 DTEEC_014011_1315_014288_1315_A01.IMG 1

Table 3.2: List of orthorectified images used for the analysis. Last row is the DTM we used, which has a pixel size of 1 m. All data products are publicly available at https://www.uahirise.org/. MY is the Martian Year.

1The Mars Year (MY) is the number of years from the northern Spring equinox of Aprill 11, 1955 (Clancy et al.,2000).

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3.2. STUDY OF RSL IN RAGA CRATER 43

3.2.2 Measurement methods

We here describe how we performed measurements. First, from the DTM we computed slope and aspect of every pixel using the algorithms im- plemented in the ArcMap^® software (Burrough and McDonnell, 1999).

These data are shown in figures (3.13B and C). Next, we selected a sample of 50 RSL where we could accurately determine origin, terminus and course. They are shown with orange lines in figure (3.13 E). For each image in table (3.2) we measured the true length (i.e. length draped on the DTM, thus corrected for slope) of all RSL. In particular, lengths were measured by drawing a polygonal chain (or polyline) over each RSL for every image. The built-in function "Add Surface Information" was then used to compute the true length of our polylines.

On the contrary, starting and ending elevation, starting and ending slope, and starting aspect of all 50 RSL were measured only on images where RSL achieved their maximum extent using tools provided by the ArcMap^® software. This choice is motivated by the facts that a) the starting elevation, slope and aspect of RSL do not change over time and b) the ending elevation and slope are meaningful only when RSL are at their maximum lengths. In particular, from each polyline we obtained its starting and ending vertices using the "Feature vertices to points" built-in function. We then use the "Add surface Information" tool to extract values of elevation, slope and aspect at locations defined by starting and ending vertices of RSL polylines.

Concerning uncertainties, we assumed an error of 30 cm in elevation given by the uncertainty in the elevation of the DTM and no error in the aspect, as we are interested only in the direction (i.e. North or North- West facing) that the slope is facing and not in the precise azimuth. Re- garding lengths, instead, we measured single RSLs multiple times and considered the biggest value between the pixel size, 0.25 m, and the vari- ance of the measurements. For images with completely faded RSL, we assume a length of 0 m and error of 0.25 m. We report our elevation, slope and aspect measurement in table (3.3) and true length measurements in table (3.4); in the latter, we indicate with "L(Ls)" the true length at solar longitude Ls, in meters, and with "MY" the Martian year. A discussion of these measurements is provided in Chapter (4). We show an example of lengths measurements for a RSL of table (3.4) in figures (3.14) and (3.15), and the starting and ending slope distributions in figure (3.16).

STUDY OF RECURRING SLOPE LINEAE ON THE MARTIAN SURFACE

UNIVERSITÀ DEGLI STUDI DI PADOVA

dipartimento di fisica e astronomia “g. galilei”

STUDY OF RECURRING SLOPE LINEAE ON THE MARTIAN SURFACE

Contents

Abstract

Sommario

Chapter 1 Introduction

1.1 The planet Mars

1.2 Water on Mars

1.2.1 Stability of water on Mars

1.2.2 Evidences of past liquid water

1.2.3 Contemporary water

Chapter 2

Recurring Slope Lineae

2.1 Observations

2.2 Formation mechanisms

Chapter 3

Data Analysis

3.1 Survey of Recurring Slope Lineae and their global properties

3.1.1 RSL catalogue and global properties

3.1.2 Potential RSL sites

3.2 Study of RSL in Raga crater

3.2.1 Dataset

3.2.2 Measurement methods